<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title>区分undirld和null</title>
	</head>
	<body>
		<script>
			var message;
			if(message)  //undiefind
			{
				alert("有")
			}
			else
			{
				alert("没有")
			}
			
			
		</script>
		
		
	</body>
</html>
